Nafion perfluorinated ionomer membrane has been regarded as a benchmark material in proton electrolyte membrane fuel cells (PEMFC) due to its excellent perm selectivity properties. However, the excessive swelling and low operating temperature window (up to 80 °C) of Nafion marked its vital shortcomings in the proton fuel cell operations.
This dissertation is focused on the modification of hydrophobic fluorocarbon matrix and hydrophilic ionic domains/clusters of Nafion to alleviate the aforementioned shortcomings. The first modification is via solution blending of Nafion with copolymer of poly(vinylidenefluoride-trifluoroethylene) or PVDF-TrFE, which modified the fluorocarbon matrix of Nafion, while the second is via simple insitu impregnation of Nafion with several kinds of functionalized supramolecules, which locally alters the properties of the ionic domains/clusters.
Nafion/PVDF-TrFE blends revealed an hourglass type phase diagram, consisting of single phase crystal (Cr1), and crystal + liquid (Cr1 + L2) and liquid + liquid (L1 + L2) coexistence regions. Blends of Nafion with PVDF-TrFE demonstrated swelling reduction upon hydration as confirmed by Fourier transform infrared (FTIR) spectroscopy and water uptake measurements. The 60/40 Nafion/PVDF-TrFE blend with bicontinuous morphology exhibits both capacitor and proton conductivity properties. This blend was found to have lower proton conductivity with a decreasing trend upon increasing temperature.
Supramolecules of photocurable hyperbranched polyester (HBPEAc-COOH) were used in the first part of in-situ impregnation attempt. Of particular importance is that the present study is the first to successfully incorporate polymer molecules/networks into the Nafion ionic domains by means of impregnation with hyperbranched supramolecules followed by photopolymerization in-situ. HB impregnated Nafion membranes were found to render swelling suppression as well as improved mechanical stability. This impregnated membrane exhibited an increasing trend of proton conductivity with increasing temperature, which eventually surpassed that of neat Nafion above 100 °C.
Two unique supramolecules terminated with hydroxyl (Noria) and tert-butyloxycarbonyl (Noria-BOC), which resembles a waterwheel, were used in the second impregnation attempt. We anticipated that these waterwheel supramolecules, Noria in particular, will have potential utility as the “solid proton carrier” in proton fuel cells to substitute the role of water at high temperatures. Noria and Noria-BOC impregnated membranes exhibited excellent swelling suppression and the mechanical stability of both impregnated membranes increased beyond the existing neat Nafion. Only Noria impregnated membrane shows improved proton conductivity at elevated temperature whereas Noria-BOC impregnated membrane revealed a plummet in the proton conductivity at high temperatures similar to that of neat Nafion.